It is often advantageous to mix a gas with a liquid reactant used in processes for producing molded microcellular elastomers to help promote uniform density, to help completely fill the mold and to help eliminate sink marks in thick molded sections.
Mixing a gas with a reactant is particularly useful in reaction injection molding ("RIM") process. The RIM process involves the high pressure mixing of highly reactive liquid reactants (e.g., polyol and polyisocyanate) in a mixing head to form a reaction mixture and injecting the reaction mixture into a closed mold, generally by using a high output, high pressure dosing apparatus. This high pressure mixing of the liquid reactants is often referred to as "impingement mixing". RIM is particularly suited for forming polyurethane articles such as resilient automobile fascia.
In RIM polyurethane processing, the polyol usually contains a number of additives such as catalysts, surfactants, cross-linking and chain-extending agents and fillers. A gas, such as nitrogen or air, is usually added to the polyol to act as a "blowing agent". The polyol (containing the additives and gas) and the polyisocyanate are separately brought to injection pressure, approximately 1800-2000 psi, and then mixed in the mixing head. The high pressure helps achieve the necessary high speed impingement mixing of the two reactants. At this high pressure, the gas begins to dissolve in the polyol. The degree of dissolution depends upon the time the polyol is held at high pressure. After the impingement mixing, the reaction mixture of the polyol and polyisocyanate travels from the high pressure mixing head to an atmospheric pressure mold. The gas expands due to the reduced pressure in the mold to fill the mold and the reactants react to form a microcellular elastomer of low density and good surface replication.
The prior art describes various means for mixing a gas (which is preferably an inert gas such as nitrogen) with liquid reactants. For example, the gas may be mixed with a liquid reactant by means of a mixing lance or by means of a fast running agitator in the storage container for the reactant. As a further example, the gas may also be added through porous metal plates or injection nozzles via a dosing device into the liquid reactant. As a still further example, the liquid reactant can be circulated from a reactant storage container and the return flow passed through the gas in a gas storage container which is under pressure and thus is absorbed by the reactant.
Various other procedures for mixing a gas and a liquid are described in the prior art. For example, U.S. Pat. No. 4,059,714 describes a process for preparing an adhesive foam by intimately mixing air or an inert gas with a thermoplastic adhesive in a liquid state and then pressurizing the liquid/gas mixture (e.g., 300 psi) to force the gas into solution in the liquid adhesive.
U.S. Pat. No. 4,156,50 describes a process for treating a body of liquid (e.g., aqueous waste material) with a gas, which process comprises passing a stream of the liquid through a conduit, injecting gas intermittently into the stream at high pressure (e.g., above 50 psi) to dissolve at least some of gas in the liquid stream, and introducing the stream containing dissolved gas and undissolved bubbles of gas into the main body of liquid under turbulent conditions such that the undissolved bubbles are shattered into even finer bubbles which dissolve in, or are consumed within, the main body of liquid.
U.S. Pat. No. 4,157,427 describes a process for entraining minute bubbles of a gas into a liquid reactant used in a RIM process for producing molded microcellular polyurethanes. This process consists of continuously removing a small portion of the reactant from a container, passing this portion through a recirculation line wherein minute gas bubbles are introduced under pressure and dispersed from a microporous stone into the flowing portion of the reactant followed by mixing in a mixing device, (e.g., a static mixer) and then returned to the container. When sufficient gas has been entrained, the reactant is delivered from the container to a mixing head, mixed at high pressure with the other reactants and injected into a mold. The entrained gas expands due to the reduced pressure in the mold to fill the mold and form a microcellular article.
U.S. Pat. No. 4,288,564 describes a RIM process for production of microcellular elastomeric molded polyurethane from liquid reactants wherein at least one liquid reactant contains a dissolved gas. An inorganic, finely divided nitrogen absorbing agent (e.g., activated carbon or iron oxide) is added to the gas-containing liquid reactant or reactants to accelerate the transition of the gas from the dissolved state to the dispersed state upon release of the pressure upon introduction of the reactants into a mold. The nitrogen absorbing agent is utilized to eliminate undesirable density variations in the molded elastomer.
U.S. Pat. No. 4,548,776 describes a process for effecting dispersion of a gas within a plastic material to facilitate molding of the plastic material in the form of structural foam. No specific plastic materials are disclosed and only the use of preformed plastic materials are described (not mixtures of reactants). This process involves an arrangement by which molten plastic material, which is to be molded and which contains a gas therein, is injected under "high pressure" (no specific pressures are disclosed) into and drives a rotatably mounted mixing turbine positioned just upstream of the mold assembly. This mixing turbine mixes the gas and plastic material to create the dispersion. This patent states that when the gas is introduced into a plastic material, the microbubbles of gas tend to migrate together to form undesirably large gas bubbles lacking uniformity of size and distribution. The further the gas is introduced into the plastic from the mold, the more the problem is exacerbated. Attempts to enhance the dispersion of the gas in the plastic prior to introduction into the mold have included placing mixing devices in the flow path of the plastic material as it travels to the mold. According to this patent, these attempts have had limited success. For example, this patent states that a static mixer is not effective for maintaining a uniform and fine dispersion of a gas in the plastic material.
It is desired (and often essential) to reduce the formation of large voids in molded microcellular elastomeric products because large voids can result in inferior or defective articles. The prior art techniques for mixing the gas with one or more of the liquid reactants used in preparing molded microcellular elastomers frequently result in the formation of large undesirable voids in the molded product. The use of various means to reduce void formation may entail other problems, even if void formation is reduced. By way of illustration, the use of nitrogen adsorbing agents to eliminate these voids is often undesirable because the agents can abrade certain high pressure metering pumps used in molded microcellular elastomer procedures and so can cause undue wear on these pumps.
Accordingly, it is an object of this invention to provide an improved process for preparing molded microcellular elastomers from two or more liquid reactants wherein at least one liquid reactant contains a dispersed gas.
More particularly, it is an object of the present invention to provide an improved RIM process for making molded microcellular polyurethane elastomers from liquid reactants.